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  • 1. Internet Applications: File Transfer Protocol (FTP)

2. Data transfer before Internet

  • Magnetic media like tapes and disks:
    • An application transferred data on magnetic media
    • The medium was physically moved from one computer to another;
    • Drawback: SLOW
  • Fax:
    • Use the telephone lines;
    • A fax machine consists of a printer, a scanner, a dial-up modem, and a dedicated computer;
    • Drawbacks: requires a dedicated machine and a fax transmission is as expensive as a phone conversation.

3. The Internet can be used to transfer data

  • Benefits:
    • Efficient: Internet is designed for sending digital data;
    • Less expensive than fax: Internet access is billed a flat rate;
    • Can transfer more types of data than fax, includingaudio and video.

4. File Transfer Protocol (FTP)

  • A general-purpose protocol that can be used to copy an arbitrary file from one computer to another;
  • one of the oldest network application---predates TCP and IP;
  • Later versions were built on top of TCP/IP;
  • Among the most heavily used applications:
    • FTP generated as much as 1/3 of the traffic on the Internet
    • Was exceeded only by WWW(in 1995).

5. Issues in designing FTP

    • Must transfer an arbitrary file (size, name,..)
    • Must accommodate multiple file types;
    • Must connect heterogeneous computers. May have to deal with different:
      • Data encodings;
      • File names;
      • File protections;

6. FTP Commands

  • FTP is an interactive protocol: it responds to each command a user enters; signals when it is ready to execute another command;
  • Examples of FTP commands:
    • Open---connect to a remote computer;
    • Get---retrieve a file from the remote computer;
    • Put---sends a file to the remote computer;
    • Bye---terminate the connection and leave FTP.

7. Transfer Modes

  • FTP defines two types of transfer: textualand binary;
  • Textual: is used for text files;
    • most textfiles are encoded in ASCII or EBCDIC
    • ftp can translate from the local to remote character set when transferring a file;
  • Binary: used for all other files (audio, image, numbers, )
    • Files are copied exactly;
    • The resulting copy might be meaningless because FTP does not convert values to the local representation;

8. Connections, authorizations and file permissions

  • The remote system has to verify that the user is authorized to access files:
    • The user has to provide a login name and a password;
    • If the user is authorized he/she may start transferring files;
  • What if the user does not have an account?
    • System administrator can configure FTP to support anonymous FTP;
    • Login name anonymous and password guest (or e-mail address) allows a user access to public files.

9. A browser can use FTP

  • A WWW browser can be used to FTPinstead of a dedicatedinterface;
  • A browser uses FTP as the transfer protocol, when the URL starts with ftp (instead of http)
  • EX:
    • --- instructsthe browser to get file as7656/temp from
    • --- displays all files in the directory as7656

10. FTP uses the client-server paradigm:

    • Local application (or browser) is the client
    • Remote FTP program is the server;
    • The FTP server authorizes the connection, locates the file, anduses TCP to send it.